Portable radio devices typically comprise an internal antenna, which means that the radiating antenna element is located within the smooth overall outline of the device, without the antenna causing any protrusions, and enclosed inside an essentially continuous outer cover of the device. The outer cover should naturally cause as little attenuation of radio waves as possible, so that it would allow the antenna to freely receive and transmit radio frequency transmissions. The radiating antenna element is typically flat and comprises conductive sections, strips and/or patches. A ground plane is needed inside the radio device and relatively near to the radiating antenna element to achieve proper operation.
A prior art publication WO 2005/034286 discloses a combined antenna and cover structure for a portable radio device. A central idea of the invention is to “bake” the radiating antenna element into the material of the outer cover, and to use a capacitive feed to couple it to the antenna port of the transceiver. A relatively similar solution is known from the publication EP 1 439 602, which mentions that the radiating antenna element may also comprise of a foil or other conductive material attached to an inner surface of the outer cover. A publication JP 2000114832 discloses an antenna structure, in which the antenna is a of the built-in planar type, although a protruding part of the outer cover is separately provided for it in order to bring the antenna away from the attenuating shadow of other components in the portable radio device. A prior art publication JP 57-79711 suggests placing the planar antenna at the outer surface of the outer cover. A yet another prior art publication U.S. Pat. No. 5,455,596 introduces various antenna modules that can be used in portable radio devices.
A problem of the known prior art antennas of the kinds described above is the effect of radio frequency losses in the cover materials. Losses in the radiating antenna element itself are typically not of importance, because it is relatively easy to make the radiating antenna element from a sufficiently thick layer of sufficiently conductive material, such as copper, so that radio frequency losses are to a large extent eliminated. The cover material, on the other hand, has traditionally been selected on other grounds than low RF losses. A vast majority of outer covers for portable radio devices are manufactured by injection molding. The material used for an injection molded outer cover must naturally have properties that are advantageous in the process. Also, the completed outer cover must have sufficient mechanical stiffness and durability as well as dimensional accuracy, and it must serve as a good basis for surface treatments such as decorative painting.
In prior art literature the problem of losses appears at most in the form of abstract statements. Publication WO 2005/034286 calls for “a material with as low losses as possible”; the publication U.S. Pat. No. 5,455,596 speaks about a “curable dielectric resin film”. It is often customary to characterize the losses of various dielectric materials with their relative electric permittivity, also designated as the dielectric constant of the material. However, a better measure of the actual losses is the so called dielectric loss tangent (tan d), which is the imaginary part of the dielectric constant divided by the real part of the dielectric constant. The dielectric loss tangent is typically frequency dependent. As an example, the commonly used low-frequency circuit board material FR-4 has a relative permittivity between 4.1 and 4.5, and a loss tangent value of about 0.02 at 1 MHz, while high-frequency circuit board materials such as DiClad® made by Arlon Materials for Electronics has a relative permittivity between 2.17 and 2.65 and a loss tangent value between 0.0008 and 0.0022 in the range from 1 MHz to 10 GHz.
The most common materials used for injection molding are acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), high-density polyethylene (HDPE), poly-methyl-methacrylate (PMMA), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC). Loss tangent values for these materials in their pure forms are found in B. Riddle, J. Baker-Jarvis, J. Krupka: “Complex Permittivity Measurements of Common Plastics Over Variable Temperatures”, IEEE Transactions on Microwave Theory and Techniques, Vol. 51, No. 3, pp. 727-733, Mar. 2003. Approximate values for frequencies around 10-11 GHz and room temperature are the following: ABS 0.006-0.009, PC 0.0004-0.0006, HDPE 0.0001-0.0002, PMMA 0.006-0.01, PP 0.00007-0.0001, PS 0.0004-0.0006, and PVC 0.005-0.008. In many cases the material used for an injection molded object is a mixture of at least two different kinds of plastic. Reinforcement materials such as glass fibers or the like can be mixed to the plastic to achieve suitable mechanical properties.
The prior art problem of losses in the dielectric outer cover material is made worse by environmental conditions, which cause e.g. moisture and impurities to get absorbed in the outer cover material, which tends to increase the original loss tangent value of the material. Yet another problem of the prior art antenna structures is that if the portable radio device is transmitting at full power, losses in the antenna structure may cause local heating, which the user feels through the outer cover. Users do not like to feel such local heating, because they easily associate it with assumed malfunctioning of the device.